1. Field of the Invention
The present invention relates to telecommunications, and more particularly, wireless communications.
2. Description of Related Art
Expanded efforts are underway to support the evolution of the Universal Mobile Telecommunications System (UMTS) standard, which describes a network infrastructure implementing a next generation Wideband Code Division Multiple Access (W-CDMA) air interface technology. A UMTS typically includes a radio access network, referred to as a UMTS terrestrial radio access network (UTRAN). The UTRAN may interface with a variety of separate core networks (CN). The core networks in turn may communicate with other external networks (ISDN/PSDN, etc.) to pass information to and from a plurality of wireless users, or user equipments (UEs), that are served by radio network controllers (RNCs) and base transceiver stations (BTSs, also referred to as Node Bs), within the UTRAN, for example.
Standardizing bodies such as the 3rd Generation Partnership Project (3GPP and 3GPP2), a body which drafts technical specifications for the UMTS standard and other cellular technologies, have introduced several advanced technologies and enhancements in an effort to ensure that any associated control information is carried in an efficient manner. Certain advanced or enabling technologies may include fast scheduling, Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat Request (HARQ) technologies. These technologies have been introduced in an effort to improve overall system capacity.
While much of the standardization to date has focused on the downlink (forward link from Node B/base station to UE/mobile station), similar enhancements are now being considered for the uplink (reverse link) to provide services such as High Speed Downlink Packet Access (HSDPA) services. Further evolution of 3G standards include the development of enhanced uplink (EU) features, which may be referred to as enhanced uplink dedicated channel (EU-DCH) services, to support high-speed reverse link packet access (uplink from mobile station to base station). Many of the techniques used in the forward link (i.e., fast scheduling, AMC, HARQ, etc.) thus may also be usable on the reverse link, so as to improve data rates, improve system capacity, and reduce system costs, for example.
A physical channel is an entity used to carry information between the physical layers, or bottom layer of the open system interface (OSI) model, at two different devices, such as a base station (Node B), mobile station (UE). The physical channel is directly transmitted over a communication media such as open air, optical fiber, etc. Currently in UMTS, there are three types of uplink dedicated physical channels employed for transmission of control information and data in the uplink: the uplink Dedicated Physical Data Channel (uplink DPDCH), the uplink Dedicated Physical Control Channel (uplink DPCCH), and the uplink Dedicated Control Channel associated with HS-DSCH transmission (uplink HS-DPCCH). These uplink dedicated physical channels are I/Q code multiplexed to provide a code multiplexed signal that is input to an amplifier for transmission on the uplink. With the development of EU-DCH services, however, new uplink dedicated physical channels, in addition to the existing physical channels, may have to be considered and/or developed to support proposed EU features.
For code multiplexing of these uplink physical channels, one concern is the potential for the peak-to-average power ratio (PAR) to increase. PAR is a ratio between the peak input power at a transmitter amplifier, such as a transmitter amplifier of a UE, for example, to the average input power to the amplifier. The peak power may be defied in terms of the average power of UEs having input power higher than a given threshold X % of the time, for example, where X is typically 99.9. PAR increases could present a significant problem for high speed data UEs utilizing both the HSDPA and EU-DCH services at the same time. In this case, the UE would be already code multiplexing the DPCCH, HS-DPCCH and potentially the DPDCH. Multiplexing additional physical channels that are needed for EU-DCH services, for example, could aggregate the PAR problem.
The effects of a PAR increase may be explained in reference to an input/output characteristic curve. An input/output characteristic curve for a given transmitter amplifier (of a mobile station or base station) is only linear within a certain region. To ensure there is no signal distortion, it is desirable to operate the amplifier within the linear region of the characteristic curve. Now, if PAR increases, this means the transmitted signal varies over a larger range. In this case, the operation point for the transmitter amplifier may have to moved lower on the characteristic curve, which reduces the output power generated by the transmitter. If no operational changes are made, PAR increases may cause the transmitter to suffer from distortion and hence, a reduction in efficiency. Thus, the cost of an amplifier may increase exponentially with the linearity of the amplifier. The wider linear range the amplifier can provide, the more costly the amplifier.